Red Stem on a plant

First, could be gentics... could be LED-related. keep that in mind as you eliminate possibilities. Did it start in petioles or stems? if it started in stems it's more likely sulfur-deficiency. if it started in petiolds (leaf 'stems), then it could be elated to P-deficiency. I've had plants that even if i boost P or S significantly it does nothing for it... i've been testing 20% higher P lately to try to get rid of teh purple petioles i often see... i think i see slight improvement, but not enough to be certain this is the cause of the improvement.... as long as i don't see toxicites, going to continue to test it out. Seems to be reduced but could easily be within nomal volatility of outcomes too. this is why it's so hard to learn things in a small garden with numerous factos that changes, even when we put in a good effort to keep things consistent.

Signs of phosphorus deficantcy but can also be down to temperature in grow space or simply genectics But I'd leane towards a deficantcy with yours as i can see yellowing tips check ph and ppm of run off against what you put in. May be over feeding or under

It’s because of stress. Growing Grannie already gave the right answer 😜